Abstract
Refractory multi-principal element alloys (MPEAs) have exceptional mechanical properties, including high strength-to-weight ratio and fracture toughness, at high temperatures. Here we elucidate the complex interplay between segregation, short-range order, and strengthening in the NbMoTaW MPEA through atomistic simulations with a highly accurate machine learning interatomic potential. In the single crystal MPEA, we find greatly reduced anisotropy in the critically resolved shear stress between screw and edge dislocations compared to the elemental metals. In the polycrystalline MPEA, we demonstrate that thermodynamically driven Nb segregation to the grain boundaries (GBs) and W enrichment within the grains intensifies the observed short-range order (SRO). The increased GB stability due to Nb enrichment reduces the von Mises strain, resulting in higher strength than a random solid solution MPEA. These results highlight the need to simultaneously tune GB composition and bulk SRO to tailor the mechanical properties of MPEAs.
Highlights
Multi-principal element alloys (MPEAs), colloquially known as “high entropy” alloys, are alloys comprising four or more elements, usually in nearly equiatomic concentrations[1,2,3,4,5,6,7]
Using this MPEA spectral neighbor analysis potential (SNAP) model, we show that the Peierls stress for both screw and edge dislocation in the equiatomic NbMoTaW MPEA are much higher than those for all the individual metals, and edge dislocations become much more important in the MPEA than that in the pure elemental bcc system
From Monte Carlo (MC)/molecular dynamics (MD) simulations, we find strong evidence of Nb segregation to the grain boundaries (GBs) of the NbMoTaW MPEA, which in turn has a substantial effect on the observed short-range order (SRO)
Summary
Multi-principal element alloys (MPEAs), colloquially known as “high entropy” alloys, are alloys comprising four or more elements, usually in nearly equiatomic concentrations[1,2,3,4,5,6,7]. They have drawn rapidly growing interest due to their exceptional mechanical properties under extreme conditions. The facecentered cubic (fcc) FeCoNiCrMn MPEA and the closely related three-component “medium-entropy” CrCoNi alloy have been reported to have high fracture toughness and strength, which is further enhanced at cryogenic temperatures[1,8]. It is clear that the microstructure (e.g., nanotwinning), short-range order (SRO), phase transitions, and other effects play significant roles[11,12,13,14]
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